The present invention relates to a liquid crystal display device, and more particularly relates to a source follower circuit constructed from a polysilicon thin film transistor (hereinafter referred to as a "polysilicon TFT" (Thin Film Transistor)) and an output circuit for the liquid crystal display device employing this source follower circuit as an output buffer.
Output buffers for charging each column line capacitor in a liquid crystal display device are generally constructed with voltage follower circuits employing operational amplifiers. However, in integrally forming a liquid crystal panel and a driver circuit thereof using polysilicon, complicated circuits for the operational amplifiers and variation in characteristics and large threshold voltage Vth of polysilicon TFTs make it difficult to form voltage follower circuits with polysilicon. This causes difficulty in integrally forming a liquid crystal panel and a driver circuit thereof with polysilicon.
It has therefore been considered to construct an output buffer using a source follower circuit of a simple circuit configuration. A simple source follower circuit configuration employing a polysilicon TFT is shown in FIG. 1. In FIG. 1, a source follower transistor 101 is in a connection used as a source follower and a drain of the source follower transistor 101 is connected to a power supply VCC and a gate is served as an input terminal. A source of the source follower transistor 101 is served as an output terminal and a current source 102 is connected across the source and ground.
In the source follower circuit of this configuration, an offset corresponding to a gate-source voltage Vgs of the source follower transistor 101 occurs across the input and output terminals. Namely, the output voltage Vout becomes as EQU Vout=Vin-Vgs.
Since the offset potential Vgs is a function of variables such as the threshold voltage Vth of the transistor and the mobility of carriers .mu. as is described later, the output voltage Vout therefore varies due to variations in transistor characteristics.
The offset potential Vgs of a source follower circuit can generally be expressed by the following equation. EQU Vgs=Vth+Iref.vertline.k)
and, k=0.5.times..mu..times.Cox.times.W/L PA1 where, Iref is current of the current source 102, k is a constant, and Cox, W and L are a capacitance of a transistor oxidation film, gate width, and gate length, respectively.
As becomes clear from the above description, variation in the Vth of a transistor is substantial even for a source follower constructed with a polysilicon TFT, so that variation in output potential is also substantial. Therefore, when this circuit is used as an output buffer for charging each column line capacitor, there are large variations in output potential between the circuits. It is therefore difficult to employ a source follower circuit of the current configuration as an output buffer as it is for an integration of a liquid crystal panel and a driver using polysilicon.
In order to resolve the aforementioned problems, it is an object of the present invention to provide a liquid crystal display device having a source follower circuit with highly precise offset cancelling and an output circuit employing this source follower circuit.